1. Field of the Invention
The present invention relates to an apparatus and method for controlling a power converter. More particularly, the present invention relates to a power converter controlling apparatus that can prevent burnout caused by overcurrent and/or overvoltage generated by low power index operation or output short circuit in a high frequency power converter employing a digital controlling method, and a method thereof.
2. Description of the Related Art
Generally, a power converter such as an inverter uses a semiconductor device, for example, Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) for power, as a switch, and it controls the amplitude of output voltage through the intermittent on/off operation. Herein, an apparatus for controlling the intermittent operation of the switch, such as a power converter controlling apparatus, can be analog or digital. An analog power converter controlling apparatus has an advantage in that it can perform control at a faster speed. However, since the analog power converter controlling apparatus has a fixed circuit, it cannot be equipped with diverse functions, as compared to a digital power converter controlling apparatus.
FIG. 1 presents an example of a system employing a power converter. It shows an image forming apparatus having a Direct Current/Alternating Current (DC/AC) inverter 30 using a digital controlling apparatus. In FIG. 1, an AC/DC rectifier 20 rectifies power source voltage and outputs DC voltage, and the DC/AC inverter 30 receives the DC voltage from the AC/DC rectifier 20, generates high voltage and provides the high voltage to a charged roller 40. Herein, a controlling apparatus 50 controls a duty rate of gate signals which control the intermittent operation of a switch in order to maintain a phase margin and a real/apparent power ratio at a uniform level based on a resonance frequency by using a Phase Loop Lock (PLL) method for increased instantaneous power control and power conversion efficiencies. The DC/AC inverter 30 is equipped with an environment checking mode for checking a minimum power transmission environment, when an output end is short. If a minimum power transmission condition is not fulfilled, the gate signals are pre-scanned at a maximum frequency.
FIG. 2 is a circuit diagram illustrating a conventional power converter controlling apparatus. Referring to FIG. 2, the output voltage or output current measured by diodes D1 to D4, a resistor R1, and a capacitor C1 at the output end of a power converter is transmitted to a digital controller 51 through rectification and AC/DC conversion processes. The digital controller 51 outputs gate signals for controlling the intermittent operation of a switch based on the transmitted signals.
However, when overcurrent flows through a MOSFET, which is used as a switch, due to output short circuit and the like, a main switch 53 providing the switch or operation power heats up. The generation of heat degrades the main switch 53 to be burnt out. Time taken for the main switch 53 to reach the burnout is in proportion to the quantity of energy. In case of a high capacity power converter, the main switch 53 can burn out within tens of minutes or seconds.
In the conventional methods, the switch or the main switch 53 is protected from heat generated from overcurrent and/or overvoltage by using a thermosensor. However, the protection of the main switch by the thermosensor or a program has a limitation in reliability and increases costs based on insulation.